Xiaoran Guo

Bio: Xiaoran Guo is an academic researcher. The author has contributed to research in topics: Precipitation & Radiosonde. The author has an hindex of 5, co-authored 5 publications receiving 103 citations.

Declining Summertime Local-Scale Precipitation Frequency Over China and the United States, 1981–2012: The Disparate Roles of Aerosols

Abstract: The local‐scale precipitation (LSP) is mainly driven by thermal convection. Here we reveal a decreasing trend in the summertime LSP frequency over both China and the United States by utilizing the hourly rain gauge data from 1981 to 2012. The contrasting aerosol trend likely contributes to this same declining trend of LSP in both countries. As aerosol optical depth (AOD) goes beyond the turning zone of 0.25–0.30, the impact of aerosol on precipitation changes from invigoration to suppression. The mean AOD is generally less and larger than this range and of opposite trends in China and United States, respectively, which likely accounts for the same declining trend of LSP hours in the two countries. The observed boomerang shape points to the importance of aerosol loading, which matters as much as, if not more than the AOD trend, thereby potentially serving as a constraint for climate model evaluation. Plain Language Summary Local‐scale precipitation (LSP) is an integral part of the freshwater cycle. Here, we show that summer LSP hours have significantly declined in the United States and China over the past three decades, a phenomenon that cannot be well explained by global warming. The relationship between LSP hours and aerosol loading is a boomerang shape; a turning zone exists for the shifting effect of aerosols from enhancing to suppressing rainfall as aerosol loading increases. China is above this zone with an increasing aerosol trend, and the United States is below it with a decreasing trend, but they have similar reductions in LSP hours. This disparate role of aerosols in the rainfall process requires holistic thinking about air pollution and climate change.

Boundary Layer Heights as Derived From Ground-Based Radar Wind Profiler in Beijing

Abstract: The vertical structure of wind is a key factor in modulating air quality, from which the determination of boundary layer height (BLH) remains a major challenge. In this paper, we developed an improved threshold method to determine the BLH from radar wind profiler (RWP) measurements. The normalized signal-to-noise ratio (SNR) profiles were used instead of the original SNR profiles to avoid instrumental inconsistencies. Additionally, a peak filter was designed to indicate the BLH based on the maximum SNR by taking into account the multiple peaks in the SNR profile. This algorithm was then applied to the RWP measurements taken in the summer (June–July–August) of 2018 in Beijing to obtain the BLHs. Validation analyses suggested that the BLH retrievals from RWP exhibited high consistency with those from radiosondes, with an average correlation coefficient of 0.69 (0.66) and a root mean squared error of 0.39 (0.41) in the daytime (nighttime). Additionally, the major features of summertime BLHs in Beijing were examined. In particular, a distinct diurnal variation in BLH was observed with a peak (1630 ± 510 m) occurring at 0600 universal time coordinated (UTC) and a minimum (587 ± 343 m) at 2300 UTC. Therefore, the algorithm presented here has great potential to be applied to other regions to obtain reliable BLHs. The findings obtained here highlight the importance of vertical wind structure in air quality studies.

The response of warm-season precipitation extremes in China to global warming: an observational perspective from radiosonde measurements

Abstract: Consensus has been reached that precipitation extremes vary proportionally with global warming. Nevertheless, the underlying cause and magnitude of these factors affecting their relationships remain highly debated. To elucidate the complex relationship between precipitation extremes and temperature in China during the warm seasons (May through September), a 60-year (1958–2017) record of hourly rain gauge measurements, in combination with surface air temperature, RH, precipitable water (PW), and convective available potential energy (CAPE) collected from 120 radiosonde stations were examined. Spatially, the scaling relationship between precipitation extremes and temperature exhibits a large geographic difference across China. In particular, the Clausius–Clapeyron (CC) and sub-CC relationships tend to occur in northwest (ROI-N) and southeast China (ROI-S), whereas the super-CC relationship is found to mainly concentrates in central China (ROI-C). Additionally, the response of precipitation extremes to temperature becomes more sensitive as precipitation intensity increases, shifting from CC to super-CC at a certain point of inflection that varies by geographic regions. This shift occurs at approximately 15 °C in ROI-C and ROI-N, but at around 20 °C in ROI-S. Within the temperature range of the super-CC slope, the PW rises with the increases in temperature, whereas the CAPE decreases with rising temperature, which is contrary to the monotonic scaling of precipitation with temperature. From the perspective of interannual variation, the precipitation extremes correlate positively with temperature. This further confirms the notion that global warming, through jointly affecting PW and CAPE, is able to considerably regulate precipitation extremes.

PM2.5 Pollution Modulates Wintertime Urban‐Heat‐Island Intensity in the Beijing‐Tianjin‐Hebei Megalopolis, China

Abstract: Heavy PM2.5 (particulate matter with aerodynamic diameter equal to or less than 2.5 mu m) pollution and urban heat island (UHI) pose increasing threats to human health and living environment in populated cities. However, how PM2.5 pollution affects the UHI intensity (UHII) has not been fully understood. The impacts of PM2.5 on the wintertime UHII in the Beijing-Tianjin-Hebei megalopolis of China are explored during 2013-2017. The results show that the UHII at the time of daily maximum/minimum temperature (UHIImax/UHIImin) exhibits a decreasing/increasing tendency as PM2.5 concentration increases, causing a continuous decrease in the diurnal temperature range. These effects are mediated via aerosol-radiation interaction (aerosol-cloud interaction) under clear-sky (cloudy) condition. The changes in PM2.5 concentration further cause different relative trends of UHII(ma)x/UHIImin/diurnal temperature range across different cities in the Beijing-Tianjin-Hebei region, which are likely related to the differences in both the PM2.5 composition and city size. This study provides insights on how air pollution affects urban climate and would help to design effective mitigation strategies. Plain Language Summary A detailed understanding of the relationship between PM2.5 (particulate matter with aerodynamic diameter equal to or less than 2.5 mu m) and the urban heat island (UHI) effect is significant for climate change adaption, planning, and sustainable development in urban regions. While the Beijing-Tianjin-Hebei (BTH) megalopolis of China is among the areas with the highest population densities and fastest urbanization rates in the world, the impacts of PM2.5 pollution on UHI, along with their regional differences in the BTH megalopolis, remain unclear. This study demonstrates that different PM2.5 concentrations in the BTH region pose various influences on the UHI intensities and their change rates in different cities of varying sizes. The UHI intensities during daytime and nighttime, respectively, exhibit weakening and strengthening tendency as PM2.5 concentration increases. These effects are mediated via aerosol-radiation interaction under clear-sky condition and aerosol-cloud interaction in cloudy weather. The relative changes in the UHI magnitudes were mainly determined by PM2.5 composition and city size. The asymmetrical influences of PM2.5 on the daytime and nighttime UHI intensities caused continuous decreases in the diurnal temperature ranges in the urban areas as the pollution level increased. Our study improves the understanding of urban climate affected by air pollution and provides a scientific basis for the mitigation of UHI impacts.

The significant impact of aerosol vertical structure on lower atmosphere stability and its critical role in aerosol–planetary boundary layer (PBL) interactions

Abstract: . The aerosol–planetary boundary layer (PBL) interaction was proposed as an important mechanism to stabilize the atmosphere and exacerbate surface air pollution. Despite the tremendous progress made in understanding this process, its magnitude and significance still have large uncertainties and vary largely with aerosol distribution and meteorological conditions. In this study, we focus on the role of aerosol vertical distribution in thermodynamic stability and PBL development by jointly using micropulse lidar, sun photometer, and radiosonde measurements taken in Beijing. Despite the complexity of aerosol vertical distributions, cloud-free aerosol structures can be largely classified into three types: well-mixed, decreasing with height, and inverse structures. The aerosol–PBL relationship and diurnal cycles of the PBL height and PM 2.5 associated with these different aerosol vertical structures show distinct characteristics. The vertical distribution of aerosol radiative forcing differs drastically among the three types, with strong heating in the lower, middle, and upper PBL, respectively. Such a discrepancy in the heating rate affects the atmospheric buoyancy and stability differently in the three distinct aerosol structures. Absorbing aerosols have a weaker effect of stabilizing the lower atmosphere under the decreasing structure than under the inverse structure. As a result, the aerosol–PBL interaction can be strengthened by the inverse aerosol structure and can be potentially neutralized by the decreasing structure. Moreover, aerosols can both enhance and suppress PBL stability, leading to both positive and negative feedback loops. This study attempts to improve our understanding of the aerosol–PBL interaction, showing the importance of the observational constraint of aerosol vertical distribution for simulating this interaction and consequent feedbacks.

Dynamic Amplification of Extreme Precipitation Sensitivity

Abstract: Significance Changes in precipitation extremes under climate change are subject to substantial uncertainty. Atmospheric moisture increases alone would make extreme rain events heavier at a well-understood rate of ∼7% K−1, but a component associated with storm dynamics is much less well-understood and can either amplify or reduce that moisture-driven intensification. This paper uses an idealized modeling framework to understand the coupling of these two components, simulating one actual heavy rain event in both the present climate and hypothetical perturbed climates. The increased heating due to increased moisture drives a dynamical increase in large-scale ascent, amplifying the moisture-driven response by as much as a factor of two for warmer climates. A useful starting hypothesis for predictions of changes in precipitation extremes with climate is that those extremes increase at the same rate as atmospheric moisture does, which is ∼7% K−1 following the Clausius–Clapeyron (CC) relation. This hypothesis, however, neglects potential changes in the strengths of atmospheric circulations associated with precipitation extremes. As increased moisture leads to increased precipitation, the increased latent heating may lead to stronger large-scale ascent and thus, additional increase in precipitation, leading to a super-CC scaling. This study investigates this possibility in the context of the 2015 Texas extreme precipitation event using the Column Quasi-Geostrophic (CQG) method. Analogs to this event are simulated in different climatic conditions with varying surface temperature (Ts) given the same adiabatic quasigeostrophic forcing. Precipitation in these events exhibits super-CC scaling due to the dynamic contribution associated with increasing ascent due to increased latent heating, an increase with importance that increases with Ts. The thermodynamic contribution (attributable to increasing water vapor; assuming no change in vertical motion) approximately follows CC as expected, while vertical structure changes of moisture and diabatic heating lead to negative but secondary contributions to the sensitivity, reducing the rate of increase.

Abnormally shallow boundary layer associated with severe air pollution during the COVID-19 lockdown in China.

Abstract: After the 2020 Lunar New Year, the Chinese government implemented a strict nationwide lockdown to inhibit the spread of the Coronavirus Disease 2019 (COVID-19). Despite the abrupt decreases in gaseous emissions caused by record-low anthropogenic activities, severe haze pollution occurred in northern China during the COVID lockdown. This paradox has attracted the attention of both the public and the scientific community. By analyzing comprehensive measurements of air pollutants, planetary boundary layer (PBL) height, and surface meteorology, we show that the severe air pollution episode over northern China coincided with the abnormally low PBL height, which had reduced by 45%, triggering strong aerosol-PBL interactions. After dynamical processes initiated the temperature inversion, the Beijing metropolitan area experienced a period with continuously shallow PBLs during the lockdown. This unprecedented event provided an experiment showcasing the role of meteorology, in particular, aerosol-PBL interactions in affecting air quality.